Dzhokhar Tsarnaev has gone on trial for his life over the Boston Marathon bombing, with his own lawyer bluntly telling the jury he did it - but arguing that he had fallen under the evil influence of his older brother.

"It was him," Judy Clarke, one of America's foremost death penalty defence lawyers, said of Tsarnaev in a startling opening statement in the nation's most closely watched terrorism trial since the Oklahoma City bombing nearly 20 years ago.

Laying out an argument clearly aimed at saving Tsarnaev from the death penalty, Ms Clarke said that the defence will not try to "sidestep" his guilt for the "senseless, horribly misguided acts carried out by two brothers".

"The evidence will not establish and we will not argue that Tamerlan put a gun to Dzhokhar's head or that he forced him to join in the plan," Ms Clarke said, "but you will hear evidence about the kind of influence that this older brother had."

She began laying out her case after prosecutors sketched out the scene of horror at the marathon and accused him of cold-bloodedly planting a bomb designed to "tear people apart and create a bloody spectacle", and then hanging out with his college friends as if nothing had happened.

"He believed that he was a soldier in a holy war against Americans," Assistant US Attorney William Weinreb said. "He also believed that by winning that victory, he had taken a step towards reaching paradise. That was his motive for committing these crimes."

A shaggy-haired, goateed Tsarnaev, 21, slouched in his seat and looked at Mr Weinreb as the prosecutor launched into his opening statement.

Three people were killed and more than 260 hurt when two pressure-cooker bombs exploded near the finish line seconds apart on April 15 2013. Tsarnaev is accused of carrying out the attacks with his older brother, now dead.

About two dozen victims took up the entire left-hand side of the courtroom, watching sombrely as Mr Weinreb described the carnage. Several hung their heads and appeared to fight back tears.

Mr Weinreb said Tsarnaev carried a bomb in a backpack, and it was "the type of bombs favoured by terrorists because it's designed to tear people apart and create a bloody spectacle".

Sketching out the horrific scene on the streets after the two shrapnel-packed pressure-cooker bombs exploded, Mr Weinreb said: "The air was filled with the smell of burning sulphur and people's screams."

The prosecutor described how eight-year-old Martin Richard stood on a metal barrier with other children so he could get a good view of the runners.

"The bomb tore large chunks of flesh out of Martin Richard," and the boy bled to death on the pavement as his mother looked on helplessly, Mr Weinreb told the jury, with the boy's parents in the courtroom.

After the bombings, Tsarnaev "acted like he didn't have a care in the world", the prosecutor said. Mr Weinreb said Tsarnaev went back to the University of Massachusetts-Dartmouth and hung out with his friends.

"While victims of the bombing lay in the hospital and learned that they would have to have their limbs chopped off to save their lives, the defendant pretended that nothing had happened," Mr Weinreb said.

Among the victims in the courtroom was Heather Abbott, who lost a leg in the attack. None of the victims came in on crutches or in wheelchairs; all appeared to walk under their own power.

Tsarnaev's lawyers have made it clear they will try to show that at the time of the attack, Tsarnaev, then 19, looked up to his older brother, Tamerlan, 26, and was heavily influenced by him. They plan to portray Tamerlan as the mastermind of the attack. He died in a shootout with police days after the bombings.

But prosecutors say Dzhokhar was an equal participant who acted of his own free will. They contend the brothers - ethnic Chechens who arrived from Russia more than a decade ago - were driven by anger over US wars in Muslim lands.

Tsarnaev faces 30 charges over the bombings and the shooting death days later of a police officer from the Massachusetts Institute of Technology. Seventeen of the charges carry the possibility of the death penalty.

Tsarnaev's lawyers fought right up until the last minute to have the trial moved out of Massachusetts, arguing that the emotional impact of the bombings ran too deep and too many people had personal connections to the case. Their requests were rejected by Judge George O'Toole Jr and a federal appeals court.

The panel of 10 women and eight men was chosen on Tuesday after two long months of jury selection, interrupted repeatedly by snowstorms and the requests to move the trial.

The trial will be split into two phases - one to decide guilt or innocence, the other to determine punishment. If Tsarnaev is convicted, the jury will decide whether he gets life in prison or death.

The trial is expected to last three to four months.

Ms Clarke, Tsarnaev's lawyer, has saved a string of high-profile clients from the death penalty, including Atlanta Olympics bomber Eric Rudolph; Unabomber Ted Kaczynski; and Jared Loughner, the man who killed six people and gravely wounded then-Representative Gabrielle Giffords in a 2011 shooting in Tucson, Arizona.

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